1) BDSM. Does. Not. Excite. Me. Sexually. Will a mixed-orientation relationship with me and a BDSM enthusiast ever work, or shall we eventually grow frustrated/bored? I'm hoping it could be a shared hobby, I'm always open to learning new stuff, but can I really trivialize someone's sexual identity to the level of a hobby?

I think it depends on how essential it is to the other person's sexuality. Like, me, I have serious submissive leanings (with certain people anyway) but I'm perfectly capable of craving and enjoying-to-pieces vanilla sex and vanilla love. Especially considering that your partner is living a poly lifestyle, there's no reason he can't have a slice of vanilla and a slice of chocolate... unless he ONLY likes chocolate, and then your vanilla self might have a problem.